Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Indubitably, you have a grasp of the problem: A door that falls off a safe, has undoubtedly had its hinge-pieces come unscrewed. And since the bolts presumably had some bushing or washer bits in with them, then you should look about for such washers or bushings (more likely washers, really, of metal) of the right diameter.
Also look on the hinges as they are: are any washers still remaining? if so, even one, remaining washer, will be a guide as to what to look for (wherever the hinges fell off, more or less), or what to look for in a store to replace them.
There might also be - or be needed - some lock-washers! since just taking the hinges off would be way too easy a way to get into the safe, unless the hinges are lock-washered on.
Also... if your safe is still less than a year old, you might just report it to Sentry Safes. For a product of theirs to fail in such a spectacular way, is news they might need to know... and they might even offer within-warranty service on your safe, to repair the hinges and make sure it doesn't happen again.
Let me know what you think... and if you need further information, too.